1. Field of the invention
The present invention relates to a process for producing copolymers with controlled morphology. More specifically, the invention relates to copolymers having blocks of alternating amorphous and semicrystalline blocks of L-lactide, D-lactide and/or (L,D)-lactide to provide unique control of the copolymer melting temperature, degree of crystallinity and crystallization kinetics.
2. Description of the Related Art
The optically active enantiomers L-lactide and D-lactide are known to polymerize with a variety of catalysts to produce the enantiomeric open-chain polymers poly(L-lactide) and poly (D-lactide), respectively. For example, H. R. Kricheldorf et al., Eur. Pol. J., Vol. 25 (1989). pp 585-591, discuss polymerization of L-lactide with Sn(II) and Sn(IV) halogenides. J. W. Leenslag and A. J. Pennings, Makromol. Chem., 188, (1987), pp 1809-1814, discuss preparation of high molecular weight poly(L-lactide) using tin 2-(ethylhexanoate) catalyst. Polymerization of L-lactide by means of magnesium salts is discussed by R. Dunsing and H. R. Kricheldorf, Polymer Bulletin, Vol. 14 (1985), pp 491-495. H. R. Kricheldorf et al., Macromolecules, Vol. 21 (1988), pp 286-293, discuss polymerization of L-lactide and other lactones with metal alkoxide catalysts. F. E. Kohn et al., J. Appl. Pol. Sci., Vol. 29 (1984), pp 4265-4277, discuss polymerization of D,L-lactide initiated with tetraphenyltin and G. Rafter et al., Acta Polym, Vol. 41 (1990), pp 611-617, presents a review on polymerization of D,L-lactide with cationic, anionic and non-ionic catalysts. The poly(L-lactide) and poly(D-lactide) are described in the literature as being semicrystalline with melting point about 175.degree. C., whereas poly(D,L-lactide) is amorphous.
Australian Published Patent Application No. 50602/90 discloses poly(ester-silicone) block copolymers wherein the polyester is formed from repeating lactide groups, repeating glycolide groups or mixtures thereof.
German Offenlegungsschrift DE 391157 discloses block copolymers wherein one type block is formed from an aliphatic polycarbonate and the other type block is a polylactone or a coploymer of a lactone and a carbonate.
Japanese Patent Application No. 59-27923 discloses poly(etheroester) block copolymers. The polyether blocks and polyester blocks are randomly arranged along the product polymer chains.
In U.S. Pat. Nos. 2,878,236 and 3,169,945, lactone polyesters are described wherein the lactone starting material is identified as having at least six carbon atoms to avoid a tendency for the resulting polymer to revert to the monomer.
The relative high melting points for poly(L-lactide) and poly(D-lactide) require relatively high processing temperatures (greater than 185.degree. C.) which translates into a very narrow processing window for such polymers (since chain scissions with decrease in molecular weight occur above 180.degree. C.). For practical purposes, lower melting temperatures (broader processing window) are desired. Traditionally, melting point depression in polylactides has been achieved by copolymerizing a controlled amount of the opposite lactide enantiomer. This method introduces lattice defects in the homopolymers which translates to less perfect crystallization and lower melting temperature. Random copolymers of the enantiomeric lactide are crystalline only when over 90% of the enantiomer is present. The melting points decrease from about 175.degree. to 124.degree. C. as composition changes from pure enantiomer to 8% of the opposite enantiomer. The decrease in the melting point of such random lactide copolymers is unfortunately accompanied by a considerable decrease in the degree of crystallinity and crystallization rate, which makes these copolymers unsuitable for many applications.
The melting point of semicrystalline polymers is also dependent on the molecular weight. A general review of this subject is given by D. Cohn, H. Younes, and G. Macrom, Polymer, Vol. 28 (1987) wherein the morphological behavior of semicrystalline polylactide with different molecular weights is described. The melting temperature (T.sub.m) and the degree of crystallinity (X.sub.c) is shown to increase with molecular weight: T.sub.m increases from 135.degree. to 170.degree. C. and X.sub.c increases from 12.2 to 37% when the number average molecular weight (M.sub.n) increases from 1,000 to 28,000 g/mole. However, no information on the crystallization rates are given in this paper.